A partially iced over evaporator can be caused by...

• A. Overcharge
• B. An undercharge
• C. Too high superheat
• D. Low system head pressure

Answer: (B)

The key idea is that frost on an evaporator forms when the coil gets very cold, below freezing, and moisture in the air freezes on contact. An undercharge means there isn’t enough refrigerant circulating through the evaporator, so the suction pressure drops and the evaporating temperature falls lower than intended. With the coil running colder than the surrounding air, moisture condenses and freezes, often frosting over part of the coil while the rest remains unfrozen. The other conditions don’t typically create that frost pattern: overcharging tends to push liquid into the compressor and raises head pressure; too high superheat means the evaporator isn’t absorbing enough heat (not the direct cause of icing); and low system head pressure relates more to the condenser side and doesn’t usually produce frosting on the evaporator. So, the partially iced evaporator is most commonly caused by an undercharge.